1. Field of the Invention
The invention relates to a method for sedimentation of sludge in waste water, which in a stream is supplied to a sedimentation basin via an inlet and in clarified condition is removed via an outlet placed opposite to the inlet.
2. Prior Art
Before waste water can be discharged to the receiver it has to be clarified to such an adequate extent that it can meet the present outlet requirements. In a typical waste water treatment plant this treatment takes place in successive process steps, where the elements of solid and suspending materials in the waste water are treated and/or removed.
The solid materials are mainly removed by means of a mechanical clarification which is followed by a sedimentation of sludge in a primary clarifier. From there, the waste water is directed further on to an aeration basin, where the contents of the waste water of different types of polluting elements are decomposed and transformed to more harmless compound as biological sludge.
Finally, the biological sludge coming from the water will be separated by sedimentation in a secondary clarifier, whereafter the treated water in most cases can be discharged to the receiver. The sedimented sludge is returned to the aeration basin, where it is a part of the biological treatment process. Surplus sludge is pumped out of the aeration basin and is drained off, whereafter it normally can be spread over the agricultural areas.
Initially, the sludge in the waste water supplied to the secondary clarifier, will normally be small primary particles, which cannot or only with difficulty be able to sediment. The primary particles will first have to be gathered to larger particles or flocs in a process, which is called flocculation and takes place when the waste water by slightly stirring will be brought to gradients of velocity at a size suitable enough to make the small primary particles collide and stick together in flocs. If the gradients of velocity are too big, the flocs will, however, decompose faster than they are formed, and if they are too small, the flocs will not be formed. A sedimentation can therefore only take place in a secondary clarifier, if the gradients of velocity in the waste water have such a size, that it is within the limits of a certain interval.
Secondary clarifiers are normally constructed as either round or rectangular basins. Each basin has an inlet to direct the waste water into the basin, and an outlet to direct the treated water out of the basin. In circular basins the inlet is placed in the middle of the basin and the outlet along the perifery. In rectangular basins the inlet is placed at one end and the outlet at the opposite end.
In both cases the waste water in the basin will flow from the inlet to the outlet at an adequate low speed in order to make the sludge being able to sediment. The sludge sedimented is, by means of a bottom scraper, transported into the opposite direction to a sludge pit, from which the sludge, as mentioned before, is pumped back into the aeration basin.
A secondary clarifier has to be able securely and efficiently to sediment the existing biological sludge in the waste water, if the outlet requirements stipulated by the authorities, are to be fulfilled under all circumstances. This is not always the case.
There can by many reasons as to a satisfactory sedimentation is not always obtained. Some of these mentioned here can be insufficient floc creation and inexpedient flow conditions in the basin. Both these phenomenons are especially significant in periods with heavy rain, where a basin can be exposed to hydraulic overload.
The waste water flows via the inlet into the basin at a comparatively high speed, which typically can be about 30-40 m/min. At such high speeds the primary articles will not be able to create flocs. Not until the current of water has been slowed down to much lower speeds, which necessarily have to exist in the secondary clarifier to enable a flocculation to take place, e.g. typically less than 6 m/min., the water current has hat to cover such a long distance that it has reached the area above the sedimented sludge blanket, where the current therefore sets the water into movement towards the outlet in the shape of an undercurrent. At the same time, in compensation for the amount of water, which the undercurrent brings along, a surface current is formed with direction from the outlet to the inlet.
This current pattern is for obvious reasons very unfortunate. The undercurrent raises the sludge already sedimented having the effect that some of it is carried away and gets mixed in the water, which has already been treated. Between the surface current and the undercurrent there is, however, a zone with sufficiently calm water allowing the sludge flocs to be created. The sludge flocs will, however, have difficulty in sinking and sedimenting, since the flocs, due to the calm character of the water, are inclined to stick together as a kind of blanket without openings for the passage of the amount of water which necessarily has to be forced out to enable the flocs to be able to sink.
The European patent publication No. 0 386 163 B1 discloses an apparatus to insert in the inlet of a sedimentation basin. The apparatus consists in principle of two, one inside the other, cylindrical walls placed concentrically and limiting a space. The waste water is directed via a tangientially placed inlet into this space, which the water after en upwards rotation movement leaves via an incision in the upper part of the cylindrical walls. The movement, which the water at the same time is describing, has the character of a whirl with radial gradients of velocity of such a size, that it allows the sludge willingly to flocculate. In order to avoid that the flocs already created will not immediately after be decomposed again by the inner turbulence in the whirl, it is necessary that the waste water meets less possible resistance when it flows through the apparatus. Therefore the water flows out of the apparatus with more or less the same contents of energy, and therefore it creates a heavy movement and turbulence in the water which is present in the outlet area. Thereby the sludge flocs, which were created during the passage in the apparatus, are decomposed and the heavy currents of water, which afterwards are created in the water of the basin, will counteract a subsequent flocculation and sedimentation.